It is known that, during a continuous casting process, the determination of the level of the meniscus of the molten steel and of the detachment point of the liquid phase from the ingot mould, i.e. the beginning of the solid skin, is one of the most difficult problems for effective and timely process monitoring.
Indeed, the beginning of the solid skin, i.e. the closed solidified metal envelope which tends to increase its thickness progressively down along the ingot mould and which contains the liquid metal still in a molten state, is formed slightly under said level, and at the wall of the ingot mould due to the forced cooling of the latter.
If the level of the meniscus is not constantly and precisely monitored to eventually adjust the flow of molten steel and the steel extraction rate, the surface level of the molten steel bath may vary also quickly; such variations frequently give rise, as known in the art, to break-downs of the surface of the solid skin, which in practice interrupt the ability of the skin itself to contain the inner molten steel without leakages.
In general, such break-downs generate drawbacks which are described in detail in International Patent Application WO 2005/037461, to which reference is made in the present disclosure; this document also quotes discloses some further documents of the state of the art and discusses their features, such as, for example, JP 11304566A2.
EP 0312799 A1 discloses a device for measuring the level of the liquid in a crystalliser which makes use of at least one transmission coil fed by a medium-frequency electrical source and of a receiving coil. Said coils are arranged within the ingot mould body and are electromagnetically coupled to a wall of the crystalliser and to the inner volume of the same.
The operating principle of the above device is based on the fact that the information concerning the level of liquid in the ingot mould derives by processing the signals generated by said receiving coil, which depend on the mean temperature of the walls of the crystalliser, which may be in turn correlated, with known means, to the level of the liquid itself.
However, this solution, although efficient in certain conditions, presents some drawbacks which cannot be overcome: firstly, the presence of at least three coils, of which one is a transmission coil and two are receiving coils, is required; this fact naturally implies not only higher costs and construction complexity of the crystalliser provided with that device, but also requires a more complex and therefore less reliable processing of the signals present in the three coils.
Moreover, and this is the main drawback of that solution, the signal generated in the receiving coils is affected by the temperature of the coils themselves which, although protected by a metallic envelope, during operation reach the temperature of the cooling liquid which is never constant and which may vary during the casting, therefore also modifying the temperature of the coils.
Since the phase between voltage and current in the two coils depends in essence on the final voltage induced on the pick-up coil (the one closest to the copper wall of the crystalliser), it may be expressed according to either the voltage VV1 of the most distant coil or the voltage VV2 of the closest coil.
In essence, the phase shift between said two voltages, which we call generally “Df”, may be expressed as:Δφ=f(VV1,VV2)
Therefore, it is understood that by varying the ohmic resistance of the coils, the respective voltages will vary, both in terms of absolute value and phase; since the physical system is implicitly non-symmetric, then the voltage variations will not be equal for the two coils.
Indeed, by assumingVV1=Asen(wt+φVV1)VV2=Asen(wt+φVV2)with A and B respective constants, the phase difference induced between the two voltages will be:Δφ=sen−1(VV1/A)−sen−1 (VV2/B)
It is therefore apparent that in case of non-perfect symmetry, there will be a phase variation also when an only ohmic variation occurs.
Finally, since said ohmic resistances depend on the temperatures of the two respective coils, which are immersed in the cooling fluid, and since the temperature of said cooling fluid may vary rapidly and in an uncontrolled manner, it logically results that the temperature and consequently the ohmic resistance of the two coils also vary, and finally the phase shift between the signals of the latter varies, which ultimately causes wrong information on the level of the liquid metal in the continuous casting.
In conclusion, since the asymmetry of the physical system is implicit within the system itself, such asymmetry is extended also to the measurement process and therefore represents a defect in the respective measurement method.
From other patents, e.g. U.S. Pat. No. 4,138,888, EP 0 192 043, U.S. Pat. No. 3,336,873, U.S. Pat. No. 6,517,604, U.S. Pat. No. 6,337,566, U.S. Pat. No. 4,647,854, EP 0 010 539, EP 0 087 382, U.S. Pat. No. 4,441,541, U.S. Pat. No. 4,529,029, solutions are known which employ coils which generate electromagnetic fields for detecting the height or level of the meniscus in a continuous casting ingot mould; however, the systems disclosed therein provide the use of at least two separate coils, and therefore suffers from the same drawbacks.
In accordance with what is stated beforehand, it is therefore the object of the present invention to realize a device for measuring the level of the meniscus of liquid steel in an ingot mould in a continuous casting process, and a related method, which overcome the above described drawbacks.
Furthermore, the device according to the invention is easily manufactured and operable with materials and components available in the art and therefore cost-effective.
These objects, with other features of the present invention, are achieved by means of a device and a method according to the appended claims.